The invention relates to a process for producing a high-strength steel strip and to a device which is suitable for carrying out the process.
In a known process for producing a high-strength steel strip, the starting point is a hot-rolled strip which has been manufactured in the conventional way and undergoes a two-stage cooling on the roll-out table. In a first stage, the austenitic strip is cooled until it is in the austenitic-ferritic mixed range and is held in that range until a desired amount of ferrite has been formed. Then, the strip is cooled at a high cooling rate in order to obtain a martensite structure in the strip. A high-strength steel of this nature is known under the name Dual-Phase steel.
One object of the present invention is to provide a process which provides greater flexibility in the production of high-strength steel. Another object which the invention seeks to achieve is that of providing a process which can be carried out using simple means. These objects and other advantages are achieved by means of a process for producing a high-strength steel strip, in which liquid steel is cast in at least one continuous-casting machine with one or more strands to form a slab and, utilizing the casting heat, is conveyed through a furnace device, undergoes preliminary rolling in a preliminary rolling device and, in a final rolling device, is finishing-rolled to form a steel strip with the desired final thickness, and, in a continuous, endless or semi-endless process, the slab undergoes preliminary rolling in, essentially, the austenitic range in the preliminary rolling device and, in the final rolling device is rolled in the austenitic range or, in at least one stand of the final rolling device, is rolled in the two-phase austenitic-ferritic range and the austenitic or austenitic-ferritic rolled strip, after leaving the final rolling device, is cooled rapidly in order to obtain the desired structure.
The process is based on a continuous, endless or semi-endless process. In a process of this nature, very good temperature control is possible both over the length, the width and the thickness of the slab or the strip. Moreover, the temperature homogeneity as a function of time is very good. A device for carrying out this process is generally equipped with cooling means, so that the temperature profile as a function of the location in the installation and/or as a function of time is also readily controllable and adjustable. An additional advantage which can be cited is that the process is particularly suitable for the use of a vacuum tundish in order to adapt the steel composition to the desired properties which are to be obtained.
Owing to the high level of temperature homogeneity, it is very much possible to carry out rolling in an accurately predictable manner in the two-phase austenitic-ferritic range. Scarcely any, or no, difference in the austenite-ferrite percentage occurs across the cross section of the strip and along the length of the strip. The conventional process can only comply with the level of temperature homogeneity which is required in order to obtain homogeneous properties to a limited extent or by means of special measures. Consequently, the high-strength steel strip manufactured in the conventional way presents inhomogeneities both in cross section and in the longitudinal direction.
One embodiment of the process according to the invention is characterized in that the strip is rolled, in the final rolling device, at a temperature at which a desired amount of ferrite is present, and in that the strip leaving the final rolling device is cooled rapidly to a temperature below Ms (start martensite) within the temperature range in which martensite is formed.
Owing to the very good level of temperature homogeneity, it is possible to set and maintain a desired austenite-ferrite ratio in the final rolling device. After leaving the final rolling device, the strip is cooled very quickly, during which cooling the austenite is transformed into martensite, resulting in a high-strength strip.
It will be clear to the person skilled in the art that it is also possible to carry out the process in such a manner that the strip is rolled entirely in the austenitic range and leaves the final rolling device as an austenitic strip. A strip rolled in this way will also exhibit a very high level of temperature homogeneity both in cross section and in the longitudinal direction. The conventional method for producing Dual-Phase steel by means of two-stage cooling can advantageously be produced on a strip of this nature.
Another embodiment of the process according to the invention is characterized in that the strip is rolled, in the final rolling device, at a temperature at which a desired amount of ferrite is present, and in that the strip leaving the final rolling device is cooled rapidly to a temperature above Ms (start martensite) and at a cooling rate at which bainite is formed. In this embodiment of the invention, a desired ratio between austenite and ferrite is again created and, owing to the good level of temperature homogeneity, is equally distributed over the strip. The selection of the cooling rate and cooling temperature means that part of the austenite is converted into bainite, between which residual austenite remains. During the subsequent deformation of the steel strip when making products, the austenite generates dislocations which provide the high-strength steel with the property of deformability. The result is a steel strip with high-strength and high-ductility. Owing to these properties, these steel grades are also known as TRIP steel (transformation induced plasticity). The steel strip is coiled in the bainite range. The entire process of bainite formation and the formation of residual austenite is dependent on alloying elements. It is therefore particularly advantageous, when producing this type of steel, to make use of a vacuum tundish, which allows the composition of the steel to be adapted so as to match the desired properties right up until the last moment before the slab is cast in the continuous-casting machine.
In order to obtain not only a good level of temperature homogeneity but also a good distribution of the deformation over the cross section of the strip, a further embodiment of the process according to the invention is characterized in that on at least one stand, preferably all the stands, of the preliminary rolling device and/or on at least one stand, preferably every stand, of the final rolling device, lubricating rolling is carried out. Lubricating rolling ensures that the reduction applied by the rollers is distributed homogeneously through that part of the steel strip or the steel slab which is situated between the rollers. EP-A-0 750 049 describes a hot rolling process for the production of a dual-phase steel. A combination of alloying with specific elements and the use of specific cooling and coiling temperatures is disclosed. There is no disclosure in this document of employing a single-line process, starting from continuously casting of liquid steel.
Similar remarks apply to the disclosures in U.S. Pat. No. 4,790,889; U.S. Pat. No. 5,470,529 and U.S. Pat. No. 4,316,753.
EP-A-0 370 575 also describes a method in which a steel strip is produced in a single line, starting from continuously casting of liquid steel. This document does not, however, disclose the production of a high-strength steel strip. Also the cooling of the strip there is performed prior to the final rolling action instead of thereafter and prior to the coiling of the steel strip.
The invention is also embodied by a device for producing a steel strip, suitable in particular for carrying out a process according to the invention, comprising at least one continuous-casting machine for casting thin slabs, a furnace device for homogenizing a slab, which has optionally undergone preliminary size reduction, and a rolling device for rolling the slab down to a strip with the desired final thickness, and a coiler device for coiling the strip, which is characterized in that a cooling device with a cooling capacity of at least 2 MW/m2 is placed between the final rolling mill stand of the rolling device and the coiler device.